1. The Field of the Invention
This invention relates to devices and methods used to make dimensional measurements using video cameras. More particularly, the present invention relates to dimensional measurements of a laser beam which is used to determine important parameters of the laser beam.
2. The Prior Art
A variety of video cameras accompanied by appropriate signal processing equipment are advantageously used to make quantitative linear dimensional measurements in scientific and industrial fields. For example, video cameras based upon charge coupled devices (CCD) and vidicon devices are commonly used to make dimensional measurements.
Cameras based upon CCD and vidicon devices are rugged apparatus with many desirable characteristics for use in making quantitative dimensional measurements. One application in which such cameras are used is in laser beam analysis and diagnostics. While lasers have become ubiquitous in commercial, industrial and scientific fields, each individual laser commonly used in such fields produces a beam of characteristic parameters. For example, laser beam parameters such as the beam's intensity profile as well as other quantitative dimensional measurements must be individually determined and adjusted in many cases.
CCD cameras in particular have become popular as a tool for conducting laser beam diagnostics measurements. The ability of the CCD camera to simultaneously measure the entire surface area of the beam and perform detailed spatial measurements makes it well suited for conducting laser beam diagnostics. These cameras are used by scientists and engineers who are either designing lasers or who are using lasers in applications where the spatial profile of the laser beam is critical.
Cameras using CCD and vidicon devices, in conjunction with commercially available digitizing and processing electronics, provide primarily two desirable features. First, they provide a picture of the beam profile so that the engineer or scientist can effectively see what the laser beam profile looks like. The fast response of such cameras and signal processing equipment provides this picture in real time and in both 2D and 3D modes. The user thus gains an insightful, intuitive perspective on the laser beam characteristics.
Secondly, the signal processing equipment, e.g., digitizing electronics, are able to extract detailed quantitative measurements on laser beam characteristics from the data provided by the camera. Such detailed quantitative measurements allow a user to precisely determine the properties of the laser beam and to make incremental adjustments and improvements in its performance.
Such video cameras, in spite of their advantages, possess certain characteristics that limit the precision with which laser beam diagnostics can be carried out. CCD cameras and vidicon cameras have some characteristics that limit their usefulness in laser beam diagnostics and in other industrial quantitative measurement applications. First, CCD cameras and vidicon camera typically have a low signal-to-noise ratio, even when the signal is approaching saturation, which causes problems in obtaining precise measurements under varying camera conditions.
Second, CCD cameras and vidicon cameras possess a measurement error resulting from a fixed baseline offset error inherent in the camera. The fixed baseline offset error exacerbates the seriousness of the low signal-to-noise ratio.
These inherent characteristics have limited the ability of commercial grade CCD cameras and vidicon cameras to make accurate measurements only when the signal produced by the camera is very close to saturation and only when the signal covers a relatively large area of the camera surface.
The problems encountered with CCD cameras and vidicon cameras are accentuated in laser beam diagnostics applications because the dimensions of the laser beam impinging upon the light sensitive surface of the camera is highly dependent upon low level intensities in the wings, or outer regions, of the beam where the signal-to-nose ratio is inherently very low and may even be less than one. In contrast, the integrated total energy in the wings can be significant due to the relatively large area of the wings of some laser beams. In cases where the wing portion of the laser beam is large or exhibits high energy the noise which is inherent in cameras, especially in the presence of baseline offset error, can create very large errors in calculated beam dimensions.
In view of the foregoing, it would be an advance in the art to provide a method and apparatus which overcomes the noted problems and provides improved dimensional measurement using commercially available video cameras.